Many types of electroless gold plating rate accelerators have been used, as reviewed in Electroless Plating:Fundamentals & Applications, edited by G. O. Mallory and J. B. Hajdu, and published by American Electroplaters and Surface Finishers Society, Orlando, Fla., 1990. This work discusses electroless gold in great detail in Chapter 15. No electroless golds based on a non-cyanide thiosulfate/sulfite system were disclosed in this work. The most common formulations of electroless golds are based on gold cyanide complexes, with the addition of reducing agents such as dimethylamine borane, formaldehyde, sodium borohydride, hydrazine, etc. Metals such as lead and thallium, highly toxic materials, are listed as plating rate enhancers in these systems. Organic stabilizers such as compounds containing N-carboxymethyl groups have been used as stabilizers to allow higher temperature operation, thus increasing the plating rate (A. Kasugai, Kokai Tokkyo Koho, 80-24914, 1980). Glycine and N,N diethylglycine have been listed as components of some gold cyanide electroless plating solutions.
The same reference reviews the state of the art of electroless silver plating in Chapter 11. None of the disclosed formulations are based on thiosulfate plus sulfite salts. No plating rate accelerators were listed as being useful for any type of electroless silver plating baths. Most of the electroless silver plating solutions are based on literature recipes such as those described in Metal Finishing Guidebook and Directory, (1993 edition) comprising silver nitrate, ammonia, and a reducing agent such as formaldehyde or a reducing sugar. A few newer formulas have been patented, such as U.S. Pat. No. 4,863,766 which discloses electroless silver plating with a bath comprising a silver cyanide complex, another cyanide compound, and hydrazine as the reducing agent. Another formulation which has been disclosed contains silver potassium cyanide, potassium cyanide and a borane compound as the reducing agent (Platino, 57 (1970), pp. 914-920). This plating solution is said to allow a plating rate of 1 micrometer/hr with some stability. However, since these plating solutions contains a large amount of cyanide ions, there is a safety problem in operation of the solutions and in disposal of waste baths, rinses, and dragout.
Electroless silver plating solutions are generally considered to be borderline catalytic electroless metals. True electroless metals such as copper and nickel can continuously build total metal thickness to indefinitely thick coatings of 25 microns (0.001 inch) or more. The freshly deposited copper or nickel is fully catalytic and remains capable of initiating further electroless metal deposition. Most electroless silver baths, by contrast, rapidly lose autocatalytic activity. The freshly deposited silver metal is rarely able to continue catalytic activity beyond 0.25 microns (0.000010 inch).
Electroless gold baths based on a non-cyanide gold salt, and a combination of thiosulfate and sulfite salts are fully catalytic but have relatively slow plating rates of 1 to 1.5 microns per hour. Electroless silver baths based on a combination of thiosulfate and sulfite salts are fully catalytic, but have relatively slow plating rates of 1 to 1.5 microns per hour. It has now been discovered that amino acids are effective rate enhancers for increasing the speed of deposition of both electroless gold and electroless silver baths based on such formulations.